decompress: conditionally remove bmi2 from context

Use an helper function, which will just return 0 in case
the feature is disabled.
Allows constant propagation and removal of dead code.

lib/decompress/zstd_decompress.c

@@ -261,7 +261,9 @@ static void ZSTD_initDCtx_internal(ZSTD_DCtx* dctx)
 #endif
     dctx->noForwardProgress = 0;
     dctx->oversizedDuration = 0;
-    dctx->bmi2 = ZSTD_cpuid_bmi2(ZSTD_cpuid());
+#if DYNAMIC_BMI2
+    dctx->bmi2 = ZSTD_cpuid_bmi2(ZSTD_cpuid())
+#endif
     dctx->ddictSet = NULL;
     ZSTD_DCtx_resetParameters(dctx);
 #ifdef FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION